Mice, guinea pigs and rabbits are immunized with enterochelin and desferrienterochelin and the outer membrane receptor for the siderophore. In addition to testing for antibody and cellular reactivity, mice are being challenged to assess the effectiveness of the siderophorin system in immunoprophylaxis in Salmonella infections. The role of transferrin as a determinant of species specificity of Salmonella types is being studied by evaluating its capacity to supply iron to various organisms growing in severe iron restriction.